The present invention relates to an inspection technology for inspecting fine patterns in a semiconductor device, a photo mask, a liquid crystal device, and the like. More specifically, the invention relates to the inspection technology for inspecting circuit patterns floated on a wafer during the manufacturing step of the semiconductor device, using an electron beam.
The inspection technology for the semiconductor device, photo mask, and liquid crystal device having fine circuit patterns is a very important technology for improvement in a manufacturing yield thereof. An outline of the technology will be described below using the inspection technology for a semiconductor wafer as an example.
The semiconductor device and a liquid crystal display device, which will be hereinafter referred to as the semiconductor device, is manufactured by repeating the step of transferring a pattern formed on the photo mask on the semiconductor wafer using a lithography process and an etching process. Whether the lithography process, etching process, and other various processes are satisfactory or not and generation of foreign matter during the manufacturing process of the semiconductor device greatly affect the yield of the semiconductor device. It is therefore important to detect abnormality and occurrence of a fault early or in advance and feedback the result of detection to the manufacturing process. For this purpose, the method of inspecting a pattern on the semiconductor wafer during the manufacturing process has been traditionally carried out.
As the method of inspecting a defect that is present on a circuit pattern on the semiconductor wafer, a defect inspection apparatus that irradiates white light onto the semiconductor wafer, and makes comparison among the circuit patterns of the same type in a plurality of large integrated circuits (LSIs) using an optical image obtained by the irradiation has been proposed and put to practical use. As the inspection method that uses the optical image, for example, JP-A-3-167456 discloses a method in which an optically illuminated region on a substrate is imaged by a time delay integrating sensor, and the obtained image of the optically illuminated region is compared with design information input in advance, thereby detecting a defect.
JP-A-9-138198 discloses an inspection method in which laser light is irradiated onto the semiconductor wafer to detect diffracted light or scattered light, makes discrimination between the diffracted light from a regular circuit pattern and the scattered light from foreign matter or a defective portion of an irregular shape, thereby detecting the foreign matter or the defective portion alone.
With finer geometries of the circuit pattern, a more complicated shape of the circuit pattern, and diversification of materials, defect detection by the optical image has become difficult. Accordingly, a method in which an electron beam image having a higher definition than that of the optical image is used to make circuit pattern comparison, for inspection, has also been proposed. As a method of making pattern comparison for inspection using the electron beam, J. Vac. Sci. Tech. B, Vol. 9, No. 6, pp. 3005-3009 (1991), J. Vac. Sci. Tech. B, Vol. 10, No. 6, pp. 2804-2808 (1992), JP-A-5-258703, U.S. Pat. No. 5,502,306, and JP-A-10-234543 disclose the method in which the electron beam having an electron beam current of 10 nA or more, which is 100 times or more the electron beam current of an ordinary scanning electron microscope (SEM) is irradiated onto a conductive substrate such as an X-ray mask, one of secondary electrons, reflected electrons, and transmitted electrons that are generated by the irradiation are detected, and an image formed from a signal of the detected electrons is compared with an adjacent comparable pattern, thereby automatically detecting a defect.
The inspection method as described above will be referred to as an electron beam inspection method. In the electron beam inspection method, an image with a higher definition than that with an optical appearance inspection method or a laser inspection method can be obtained. Detection of minute foreign matter or a defect on a fine circuit pattern is thereby possible. In addition to that, it is also possible to detect conductivity or non/conductivity of the circuit pattern and an electric defect using potential contrast. The potential contrast indicates a surface potential difference which is caused by the influence of charge by electron beam irradiation and reflects the emission efficiency of the secondary electrons. The conductivity or non-conductivity of the circuit pattern and the electric defect such as a short circuit of wiring or a transistor is generated on the surface or the lower layer of the semiconductor wafer. The potential contrast and a technology that utilizes the potential contrast are described in the “Electron and Ion Beams Handbook” (THE NIKKAN KOGYO SHIMBUN, Ltd), pp 622-623.
By applying the optical appearance inspection, the optical inspection method, and the electron beam inspection method to various minute circuit patterns of the semiconductor device or the like, detection of various defects that could not be detected or discriminated by the shape of the semiconductor wafer surface as well as defects such as foreign matter and a defective pattern shape have become possible. Such defects include an electric defect such as an open circuit or a short circuit in various transistors, and a conduction fault of an opening pattern.
In the conventional electron beam inspection, the electron beam is irradiated during an inspection. Then, the secondary electrons or the reflected electrons generated by the irradiation are detected and converted into a signal, for the inspection. Irradiation of the electron beam is thus continued during the inspection. For this reason, when the surface of the wafer to be inspected is made of an insulating material and is easily subject to the influence of charge, or when a structure floated from the substrate is formed on the wafer to be inspected and charged electrons tend to be accumulated on the floated structure, the charged electrons resulting from the charge are accumulated on the insulating material during the process of the inspection. The charged potential of the surface of the wafer will therefore be changed from an initial state. When the charged potential is changed, the focusing position or irradiating position of the electron beam will be changed. Thus, a magnification for an electron beam image obtained during the inspection does not become accurate. Further, a positional drift or a focusing deviation is generated, so that the quality of the electron beam image obtained during the inspection will be changed. Hence, it has become difficult to continue the inspection with the same sensitivity and accuracy as those in the initial stage of the inspection.
Assume that an electron beam irradiating condition is changed in conjunction with the charged potential of the surface of the wafer to be inspected, when the charged potential changes. Then, the same electron beam image quality can be obtained. For this purpose, it is necessary to measure the charged potential of the wafer to be inspected real time during the inspection. It is therefore difficult to measure the charged potential real time based on the electron beam image alone. Further, it is possible to obtain the electron beam image of the same quality by correcting the focus and irradiating position of the electron beam irradiation whenever the quality of the electron beam image is changed during the inspection. However, when frequent suspension of the inspection and frequent focusing and positional alignment are performed on the wafer to be inspected, the time required for the inspection will become longer. Hence, speeding up of the inspection has become difficult, which leads to an increase in the manufacturing cost of the semiconductor device.
Further, by setting an inspecting condition that causes a less change in the charged potential when the inspecting condition is set, the inspection of the semiconductor device using a stable image becomes possible. However, to do so, a method of performing the inspection for a long time using the set condition, checking presence or absence of a drift in the image, and obtaining an optimal inspecting condition was employed. An enormous time was therefore required for determining the optimal inspecting condition.
In the conventional electron beam inspection, the electron beam is irradiated onto the wafer to be inspected during the inspection. Then, the secondary electrons or the reflected electrons generated by the electron beam irradiation are detected and converted into a signal, for the inspection. Irradiation of the electron beam is thus continued during the inspection. For this reason, when the surface of the wafer to be inspected is made of an insulating material and is easily subject to the influence of charge, or when a structure floated from the surface is formed on the wafer to be inspected and charged electrons tend to be accumulated on the floated structure, the charged electrons are accumulated on the insulating material during the process of the inspection. The quality of the electron beam image is not therefore stabilized. The conventional arts described above did not refer to a method of addressing this problem, or the method of obtaining the image without changing the charged state of the wafer during the inspection.